1. Field of the Invention
The present invention relates to a light source unit adapted for use in a lighting apparatus having light-emitting devices, such as light-emitting diodes (LEDs), and the lighting apparatus using the light source unit.
2. Description of the Related Art
A light-emitting device such as an LED has properties that its light output and service life are reduced as its temperature increases. For a lighting apparatus that uses solid-state light-emitting devices, such as LEDs or EL devices, as its light sources, therefore, it is important to suppress the temperature increase of the devices, in order to extend or improve the service life or luminous efficiency of the apparatus. A lighting apparatus using LEDs as its light sources is disclosed in Jpn. Pat. Appln. KOKAI Publication No. JP2006-172895A. In this lighting apparatus, a substrate is attached to a mounting plate capable of heat dissipation. The mounting plate is fixed to a main body of the lighting apparatus at mounting portions that are located in a point-symmetric manner on the peripheral edge of the body. Heat generated in the substrate is transmitted to the main body of the lighting apparatus via the mounting plate. Thus, the heat-discharge rate of the substrate is improved.
In the lighting apparatus described in Jpn. Pat. Appln. KOKAI Publication No. JP2006-172895A, however, the heat is transmitted from the peripheral edge of the substrate to the main body. The heat production in and radiation from the substrate are balanced in a certain time after the light sources are turned on. Thus, the temperature distribution of the substrate is generally uniform.
Immediately after the light sources are turned on, however, the temperature of a central portion of the substrate is liable to increase. If the light sources are repeatedly turned on and off in this condition, the irregular temperature distribution immediately after the lighting causes a reduction in the service life or properties of the light-emitting devices mounted on the central portion of the substrate. For example, the luminance of the light-emitting devices mounted on the central portion of the substrate inevitably becomes lower than that of the devices on a peripheral portion. Primarily, moreover, heat generated in the central portion of the substrate cannot be easily radiated without regard to the elapsed time after the light sources are turned on, which is another provocative condition for temperature increase.